US2255692A - Television signal-translating system - Google Patents

Description

J. C WILSON TELEVISION SIGNAL-TRANSLATING SYSTEM Filed Jan. 27: 1959 MNM I Sept, 9, 1941.

Patented Sept. 9, 1941 UNITED STATES APATENT CFICE l'.rrgmivrsrom siziLI-msmmo John C. Wilson, Bayside, N. Y., asaignor to Hagel.

tine rporation, a corporation of Delaware Application January zr, 1939, serai No. :53.040 s claims. (ci. 17a-.7.3)

This invention relates to television systems and particularly to the video-frequency signal-translating portion thereof. 'I'he invention is especially concerned with controlling the characteristics of such systems relating to gradation and contrast oi' the illumination between the incremental areas of transmitted and reproduced images.

It is frequently desirable in television systems to introduce predetermined distortion of videofrequency signals, for example, nonlinear amplification, over certain portions of their amplitude range, in order to obtain apparent optimum iidelity of reproduction of the images being transmitted and received. The desirability of such distortion is due to certain inherent characteristics of television apparatus as well as to certain physiological phenomena of the observer. In conventional television apparatus, for example, certain of the` component elements, such as amplifiers, cathode-ray tubes, and the like, have inputoutput or stimulus-response characteristics which deviate from linearity to such an extent as to result -in appreciable distortion in the translated signal unless compensated for by the introduction of predetermined complementary distortion. Moreover, in some instances, if the illumination of the incremental areas of the reproduced image is proportional to the illumination of the corresponding areas of the original image, that is, if the system has a linear over-all stimulus-response characteristic, the reproduced image may appear ilat'V or distorted to an observer. In such cases, therefore, it is desirable to give the system a predetermined nonlinear over-all response characteristic.

In photography, a iilm is said to have a gamma which deviates from unity in accordance with differences in the relative detail or contrast for the brighter or darker portions of the scene represented thereby, compared to other portions thereof, with respect to the corresponding contrasts of the original scene. This same concept is useful in television. Briefly, the gamma of anyreproduction may be defined as the slope of the stimulus-response curve plotted on a logarithmic scale. Obviously, only Where gamma is unity is response curves obtained have power law rdistortions rendering them parabolic; that is, they have a deviation in their slopes with respect to their slopes near the origin except 'for the one condition that the linearly-responsive system has a slope of unity. By transposing these curves to logarithmic scales, they all become linear, only the curve representing a linear relationship still having a slope of unity. The slopes of these latter curves. therefore, provide convenient references to the corresponding parabolic characteristic curves acteristic is such that it effects a contraction of a portion of the amplitude range of the signal translated thereby relative to the portion of` such amplitude range near its origin; and its gamma is unity only when its transmission characteristic is linear over the entire signal-amplitude range.

It is highly desirable, therefore, that gammacontrol means be provided for television signaltranslating systems in order to compensate for the various inherent nonlinear-response characteristics of such systems and the physiological phe- V nomena referred to above.

there a linear relationship between the stimulus and response over the entire response range ci?l the system.

While the stimulus-response characteristic of a linear system on a linear scale is, of course, a straight line through the origin, in television systems 'it is conventional to plot the coordinates representing stimulus in terms of the minimum It is an object of the present invention, therefore, to provide an improved system for effecting predetermined distortion of a television videofrequency signal whereby improved contrast effects over a selected portion of the signal-amphtude range may be procured.

It is a particular object of the present invention to provide an improved gamma-control means for television signal-translating systems.

In accordance with the present invention, a television video-frequency signal-translating system comprises a signal-translating means, means for applying to the signal-translating means a television signal including video-frequency and unidirectional background-illumination components for translation thereby, and means for separately applying the signal to the translating means instantaneously to modify the signaltransfer ratio thereof in accordance with the instantaneous values thereof. In accordance with appreciable stimulus, and the resultant stimulusthis arrangement, there is effected a predeter.

of which they repremined distortion of the wave form of the translatedvsignal. The system also comprises an output circuit for the translating means including means for developing therein video-frequency and background-illumination components comprising the distorted translated signal.

For a better understanding of the invention, together with other and further objects thereof, reference is had to the following description taken in connection with the accompanying drawing, and its scope will be pointed out in the appended claims.

In the drawing, Fig. 1 is a schematic diagram of a complete television signal-receiving system including a video-frequency amplifier embodying the present invention, while Figs. 2 and 3 are graphs illustrating certain operating characteristics of the system of Fig. 1 to aid in the understanding of the invention.

Referring now more particularly to Fig. 1, the system illustrated comprises a television receiver of the superheterodyne type including an antenna system Iii, II connected to a radio-frequency amplifier I2 to which there are connected in cascade, in the order named, an voscillatormotlulator I3, an intermediate-frequency ampliner I4, a detector and A. V. C. supply I5, a videofrequency amplifier I6, a second video-frequency amplifier Il embodying the present invention, and an image-reproducing device I8. A line-frequency scanning-wave generator I9 and a fieldfrequency scanning-wave generator 20 are also coupled to the output circuit of the detector I and to the scanning elements of the imagere producing device. The stages I2-20, inclusive, excepting the video-frequency amplifier I1, may all be of conventional well-known construction so that detailed illustrations and descriptions thereof are unnecessary herein.

Referring briefly to the general operation of the receiving system just described, a television signal intercepted by the antenna I0, II is selected and amplified in the radio-frequency amplifier I2 and supplied to the oscillator-modulator i3 wherein it is converted into an intermediate-frequency signal which, in turn, is selectively amplified in the intermediate-frequency amplifier I4 and delivered to the detector I5. The modulation components of the signal are derived by the detector I 5 and supplied to the video-frequency amplifier IB wherein they are amplified and supplied to the further video-frequency amplifier I1, which translates the signal in accordance with the appara-tus of the present invention, as will be presently further described, and from which they are applied to a scanningbeam control element of the image-reproducing device I8. The intensity of the scanning beam of the device I8 is thus modulated or controlled in accordance with the video-frequency voltage impressed upon its control element in the usual manner. The modulation signal is also applied to the generators I9 and 20 and the synchronizing components of the signal are utilized therein to synchronize the operations of these generators with the corresponding apparatus at the transmitter. Saw-tooth current or voltage-scanning waves, generated by the generators I9 and 20, are applied to the scanning elements of the device I8 to produce scanning fields, thereby to deflect the scanning beam in two directions normal to each other so as to trace successive series of parallel lines or fields on the target of the image-reproducing device I8 to reconstruct the image. A control-bias voltage developed by the A. V. C. supply I5 and proportional to the average carrier amplitude independent of light modulation il supplied, in the usual manner, to the control grids of one or more of the tubes in the stages l2-I4, inclusive, for maintaining the signal-input amplitude to the detector I5 within a relatively narrow range for a wide range of received signal intensities.

Referring now more particularly to the portion of the system of Fig. 1 embodying the present invention. for the purpose of controlling the contrast of the iine detailed structure of certain por.. tions of theamplitude range of the illumination of the reproduced image relative to that of other portions thereof, the video-frequency signaltranslating stage or amplifier I 1 is designed in accordance with the present invention and comprises signal-translating means, for example, a multigrid vacuum-tube amplifier 2l, preferably of the hexode type. as shown. A signal-input circuit is provided for the tube 2i comprising a volt age-divider resistor 22 included in the output circuit of the video-frequency amplier I8. The first grid or signal-input electrode of the tube 2| is coupled to the input circuit by way of a coupling condenser 23 and leak resistor 24 and an adjustable tap on the resistor 22. 'I'he third grid of the tube is similarly coupled to the input circuit by way of a coupling condenser 25, a leak resistor 26, and a second adjustable tap on the re-v sistor 22. Suitable biasing batteries 21 and 28 -are connected in series with the resistors 24 and 26, respectively. A load resistor 29 is included in the anode circuit of the tube 2i to which the input circuit of the signal-reproducing device Il is coupled; for example, the upper end of the resistor may be directly connected to the brilliancy-control electrode oi' a cathode-ray tube where such a tube constitutes the image-reproducing device. Operating potentials are applied to the electrodes of the tube 2I from suitable V sources, indicated at |Sc and +B.

For the purpose of restoring the direct current background-illumination component of the signal supplied to the amplifier I 1, which component may have been lost during previous translation of the signal, for example, by its passage through coupling condensers 23, 25 or coupling condensers in the amplifier I6, to stabilize the signal with respect to a predetermined characteristic level as applied to the control electrode of the imagereproducing device I8, rectifying means is included in the input circuit of each of the control electrodes of tube 2l. These rectifying means comprise diodes 30 and 3| having load circuits comprising the resistors 24 and 26, as shown. Where the direct current components are in the signal supplied to the tube 2|, the rectifying means are unnecessary.

In the operation of the system, the signal supplied from the amplifier I6 appears across the resistor 22 with such polarity that increasing negative signal voltage corresponds to increased illumination in the image represented thereby. The batteries 21, 28 provide initial fixed biases for the firstand third grids of the tube 2I to determine the points of operation on the characteristics of the tube. The diodes 30 and 3| function to derive from the signal input to the control electrodes of the tube 2| negative unidirectional-bias voltages equal to the peak value of the signal on the black side of its zero axis, which voltages appear across the resistors 24 and 26, respectively, and are thus applied to the first and third grids of the tube 2l to supplement their initial xed biases. The resultant signals applied to the control grids, therefore, include fixed-bias voltages translated signal.

and the direct current background-illumination signal omponents, as well as the videoor high- Ireque cy picture-signal components, and they are so applied to their respective grids that signal levels corresponding to black correspond to approximately zero voltage on each of the two control grids. Preferably, the taps on the resistor'22 are so adjusted that the amplitude of the signal voltage applied to the third grid is substantially greater than that applied to the first grid; a ratio of 3:1 may, for example, be satisfactory.

In Fig. 2, the abscissae represent signal voltages applied to the first or signal-input grid of tube 2|, and the ordinates represent anode current of the tube 2|, the curve V illustrating the applied first grid voltage. 'I'he curves as represent the mutual conductance characteristics of the tube for different voltages applied -to the third or mutual conductance controlling grid, the values given to ya being in terms of any arbitrary unit rather than voltage values. 'Ihese curves intercept the zero axis approximately ata common cutoff point, indicated at ee. corresponding approximately to a signal level representing white in the applied nrst grid voltage. Since the third grid voltage varies instantaneously in accordance with the signal applied thereto, curve A represents the effective or dynamic mutual conductance characteristic of the first grid to the anode of the tube 2|. Hence, the tube 2| has a resultant nonlinear input-voltage transconductance characteristic and constitutes a self-modulating signal-translating device.

The signal applied to its first grid is thus translated or repeated by the vacuum tube 2| andthe signal applied to its third grid varies the mutual conductance of the 'tube or its signal-transfer ratio in accordance with instantaneous values of the signal, as illustrated by curve A, to effect a predetermined distortion of the wave form of the The arrangement described provides a gamma for the amplifier l1 of a value greater than unity, for example, 2 and the output circuit of stage therefore, comprises means for developing therein the video-frequency and background-illumination components comprising the distorted translated signal. This gamma or distortion is apparent from the curve A which has a deviation or increase in its slope over the upper portion thereof with respect to the slope in the region near its origin or over the lower portion thereof. Since the upper portion of the curve in this case corresponds to the portions in `the amplitude range of the applied signal representing the darker areas of the image, the contrast of the darker area portions of the signal is expanded. That is, the contrast between the relatively dark incremental areas 0f the reproduced image is effectively increased with respect to the contrast between the relatively light areas. Curve B of Fig. 3 is identical with curve A of Fig. 2 except that it is plotted on a logarithmic scale so that the parabolic curve A is changed to the linear curve B.' showing -the gamma of th`e system, that is, the slope of curve B, which in this case is 2.

With the input signal to the amplier i1 stabllized on the peaks of the synchonizing pulses, as shown, these pulses will share in the gamma change or distortion effected by the system of the invention. In some systems, therefore, it may be preferable to stabilize the signal at the black level and remove the synchronizing pulses before they are applied to the amplifier l1.

, It will be appreciated that the present inven- 1/ tion may be readily. utilized to procure other forms of wave distortion and other gammas, it being necessary, in the system described, simply to apply the two signals to the two control grids of the tube 2| in the proper manner. Thus, the polarity of the signal applied to the first grid may be reversed with respect to that in the system described above and this signal may be stabilized with respect to a level thereof corresponding to white and so applied that this white level applies zero voltage on the rst grid, with increasing negative voltage corresponding to decreasing illumination. In this case, therefore, while the same gamma is effective for the system, the portions of the signal-amplitude range corresponding to the brighter or more illuminated areas of the image are expanded. Further, the signal may be applied to the third grid with its polarity reversed with respect to that rst described, and it may be suitably stabilized with respect to its white level and applied so that the white signal level eiiects a zero third-grid voltage. In such case, the mutual conductance characteristic of the system is so varied as to effect a gamma for the system which is lessthan unity. That is, the slope of the upper portion of the grid voltageanode current characteristic then deviates from its slope in the region of its origin in an opposite sense with respect to the corresponding deviation of curve A.

While there has been described what is at present considered to be the preferred embodiment of this invention, it will be obvious to those skilled in the art that various changes and modications may be made therein without departing from the invention, and it is, therefore, aimed in the appended claims to cover all such changes and modifications as fall within the true spirit and scope of the invention.

What is claimed is:

1. A television video-frequency signal-translating system comprising, a signal-translating means, means for applying to said translating means a television signal including video-frequency and unidirectional background-illuminay translated signal, and an output circuit for said ance With the instantaneous values thereof, thereby to effect a predetermined distortion of the wave form of the translated signal, and an output circuit for said translating means including means for developing therein the video-frequency and background-illumination components comprising the distorted translated signal.

comprising, signal-translatingl 3. A television video-frequency signal-translatins system comprising a vacuum tube having a cathode, an output electrode, and a plurality of signal-input electrodes, means for applying to one of said input electrodes a television signal including video-frequency and unidirectional back-a ground-illumination components for translation by the tube, means i'or applying said signal to' another of said input electrodes instantaneously to vary the mutual conductance of said one of w

Images